Bleaching fluffed mechanical wood pulp with hydrogen peroxide



United States Patent 3,492,199 BLEACHING FLUFFED MECHANICAL WOOD PULP WITH HYDROGEN PEROXIDE Robert R. Kindron and Frederick J. Rosebush, Pennington, N.J., assignors to FMC Corporation, New York, N.Y., a corporation of Delaware No Drawing. Filed Oct. 4, 1966, Ser. No. 584,078 Int. Cl. D21c 9/16 US. Cl. 162-71 7 Claims ABSTRACT OF THE DISCLOSURE A method for simultaneously drying and bleaching mechanical wood pulp high in non-cellulose ingredients is provided which comprises introducing the pulp at a solids concentration of 20 to 50% by weight, fluffing the pulp to provide separate to loosely agglomerated pulp fibers, introducing an aqueous alkaline hydrogen peroxide solution into the fiuffed pulp, passing the treated fluffed pulp together with a gas at a temperature of 500 to 1000 F. and substantially free of reducing agents into a drying and bleaching zone and contacting the treated fluffed pulp in the drying and bleaching zone with the heated gas stream for 2 seconds to 10 minutes to bleach the pulp and dry it to a solids content of 65 to 95% by weight.

This invention relates to the processing of mechanical wood pulps, and particularly to the simultaneous and rapid drying and bleaching of such pulps.

Mechanical wood pulps, typically those prepared by the so-called stone groundwood and refiner processes, are produced in large quantities. These pulps are especially desirable because they are economical to produce, not requiring expensive chemical pulping treatments, and because the substantial amounts of noncellulosic components of the wood in large part are retained in the pulp and not removed as is the case With chemical pulping methods.

Recently use of the mechanical pulps has expanded. Whereas in the past the bulk of these pulps were used at the location where produced, the mechanical pulps are now frequently being shipped substantial distances before use. In order to reduce shipping costs the amount of water in the pulp as it is produced, often as much as 250% of the dry weight of the pulp, must be reduced and methods have been devised which rapidly dry the pulp to essentially 65 to 95% solids. (Reference to pulp solids contents herein is to oven dried solids. This value is determined by drying the pulp to a constant weight in a circulating air oven at 105 C.) A very effective method involves mechanically pressing water out to provide a pulp solids of 20 to 50%, flufling the mechanically dried pulp to a subdivided or loose condition, and drying the pulp in a gas heated dryer to 800 to 900 F. In several seconds to 5 minutes to a solids content of to This method has a very serious drawback. Tne noncellulosic materials in the pulp cause it to darken at the drying temperatures employed, and ultimately costly bleaching processes must be carried out on the pulp to cause the pulp to have a satisfactory brightness for many end uses.

It is therefore an object of our invention to provide a method of rapidly drying mechanical pulps without darkening them.

It is a further object of our invention, not only to avoid darkening of the mechanical pulps during drying, but also to increase their brightness during the drying stage.

We accomplish these highly desirable objects by providing a mechanical wood pulp in aqueous system at a solids content .of 20 to 50% by weight and a water content of 80 to 50% by weight, flufiing the pulp to provide essentially separate to loosely agglomerated pulp fibers, introducing an aqueous alkaline solution containing 0.1 to 4%, and preferably 0.5 to 2%, of hydrogen peroxide based on the dry weight of the pulp into the fluffed pulp, passing the pulp containing the hydrogen peroxide together with a gas which is at a temperature of 500 to 1000 F. and is substantially free of reducing agents into a drying and bleaching zone, contacting the fiuffed pulp containing the hydrogen peroxide bleaching agents in the drying and bleaching zone for 2 seconds to 10 minutes, and preferably 10 seconds to 5 minutes, to dry and bleach the pulp, and removing the resulting bleached and dried pulp at a solids content of about 65 to During the drying process the heating gas drops in temperature to about to 250 F.

It is quite surprising that this process improves the brightness of the mechanical pulp. The non-cellulosic components of mechanical pulps are known to have a decomposing effect on active oxygen bleaching agents, particularly at elevated temperatures. Despite this known characteristic of such pulps, our new process very effectively bleaches mechanical pulps containing lignin and other non-cellulosic materials with active oxygen compounds, in the presence of a gas stream which contacts the pulp at an extremely high temperature in the range of 500 to 1000 F. Even more surprisingly, the pulp gains even more in brightness following its removal from the dryer. We have determined that the pulp advances in brightness on storage, for example gaining several brightness points on storage for from 1 to 30 days. On the contrary, when a mechanical pulp is bleached before drying, for example at 12% solids content with alkaline hydrogen peroxide, it actually loses brightness upon being dried at 500 to 1000 F.

The pulps treated by our process are the socalled mechanical pulps. These pulps typically are prepared by the groundwood process in which logs are ground on a grinding stone to provide the pulp, or by the refiner process in which wood chips are shredded into pulp by attrition mills or disc refiners. These pulps generally contain about 30 to 50% of lignin and other non-cellulosic materials. Chemical pulps, on the other hand, generally contain less than about 5 to on the order of 12% of such non-cellulosic materials. Mechanical pulps are normally prepared from the coniferous and deciduous woods, for example spruce, fir, hemlock, pine, poplar, cottonwood and aspen, and these various pulps all are dried and bleached effectively by our process.

As produced, the mechanical pulps are in the form of slurries in water. Such slurries have solids concentrations of about 1 to 30%. Where they are to be used at the pulp mill to produce end products, for example, paper, boxboard and the like, they can be dewatered to a solids content of 6 to 30% and bleached with active oxygen bleaching agents, and in some cases with chlorine-based bleaching agents, and the like, and then used.

However, where they are to be shipped before use, economical operation requires that they be dewatered and dried to higher concentrations, for example 65 to 95% solids, before shipping. This is accomplished by mechanically expressing water to provide pulp at a solids concentration of 20 to 50%, and thereafter rapidly drying the pulp to the desired solids concentration of 65 to 95%.

The mechanical dewatering step is readily carried out in any equipment which mechanically expresses water from the pulp slurry. Typical kinds of apparatus for use in dewatering are filters, vacuum thickeners, pulp presses, Wet lap machines and the like.

In order that the drying operation may be carried out efiiciently the dewatered pulp is fiuffed, or broken up into essentially free or loosely agglomerated pulp crumbs prior to being contacted with the heated gas. The flufiing operation is conveniently carried out in a disc mill, a hammer mill, rod mill or the like, in which the nodules or tightly agglomerated bundles of fibers which are formed in the mechanical dewatering step are subdivided. The fiufr'ed pulp resulting from this operation can be brought into intimate contact with the drying gas.

The drying step is carried out by contacting the pulp with a heated gas, generally oil or gas combustion products, or hot air introduced at a temperature of 500 to 1000 F. Such treatment drys the pulp to 65 to 95% solids in as little as about 2 seconds to 10 minutes. Obviously at higher temperatures in our range the drying takes place at shorter times, and vice versa. The gas and pulp are contacted in any zone which provides for passage of the gas over and through the fluffed pulp and for removal of the gas and the water vapor resulting from drying.

It is important that the heating gas be substantially free of reducing agents such as the 80;, formed when some fuels are burned. The high sulfur contents of such oils as Bunker C petroleum-derived fuel oil causes the formation of sufiicient sulfur dioxide to destroy the hydrogen peroxide used in our process.

In our preferred manner of operating we introduce our bleaching agent into the mechanically dewatered pulp during the flut'fing operation. Alternatively it may be introduced before or after fiufiing provided it is present on the pulp during the heating stage. It can be introduced in liquid form or as a mist or vapor, at any desired concentration which will provide the amount of bleaching agent necessary for bleaching.

Our bleaching agent is hydrogen peroxide, which is assisted in its bleaching action by an alkali, preferably a polyphosphate such as sodium or potassium tripolyphosphate, a sodium or potassium pyrophosphate, metaphosphate or the like. Other alkalies, for example trisodium or tripotassium phosphate or other alkali orthophosphate, sodium silicate, sodium hydroxide or sodium carbonate, or combinations of alkalies, may be used. However, use of sodium hydroxide or sodium carbonate alone is not recommended, because at the high temperatures used in our process these strong alkalies tend to speckle the pulp, that is, to cause localized or spotty darkening of it.

The hydrogen peroxide may be introduced as such, or as sodium peroxide, sodium perborate or other source of active oxygen which forms an aqueous alkaline hydrogen peroxide solution. It is used in an amount of 0.1 to 4%, and preferably of 0.5 to 2%, based on the dry weight of the pulp. That is, 0.1 to 4 grams, and preferably 0.5 to 2 grams, per 100 grams of dry pulp, expressed, as hydrogen peroxide. Use of more is not harmful, but is not justified economically.

The alkali is used in an amount of about 0.5 to 5% based on dry pulp weight. When sodium peroxide is used, it supplies alkalinity which should be taken into account when determining the desired alkalinity of the system. Additional bleaching aids may also be used in our bleaching system. For example, sequestering agents such as ethylene diamine tetraacetic acid, diethylene triamine pentaacetic acid or their salts, and other additives commonly used in bleaching may be employed.

By our process We provide a real increase in the brightness of mechanical pulps. Whereas typical mechanical pulps have brightnesses of on the order of 55 to 65%, when they are dried rapidly by contact with a heated gas they often lose brightness to the extent of 3 to 4 brightness points. Reference to brightness points herein is to percentage points on the well-known GE brightness scale. We are able to avoid this darkening, and even to brighten the pulps despite the fact that we contact them with very hot gases.

The eifeciveness of our process is demonstrated in the following examples, which are presented here by way of illustration of our invention and not by way of limitation of it. In these examples, brightness measurements were carried out on a Gardner Automatic Multi-purpose Reflectometer, and converted to GE brightness percentage points which are reported. Percentages referred to herein are percentages by weight, and are based on dry weight of pulp.

Example 1 A stone groundwood pulp comprising 23% of spruce, 60% of northern fir and 17% of hemlock having a brightness of 63.0%, was obtained as a slurry in Water. This pulp was dewatered and mechanically thickened to a solids concentration of 40% and fluffed or disintegrated into discrete particles resembling moist breadcrumbs, in a disc refiner. During the treatment in the refiner an aqueous solution of 1.0% of hydrogen peroxide and 1.0% of sodium tripolyphosphate was introduced into the pulp in an amount to provide 1% by weight of hydrogen peroxide and 1% by weight of sodium tripolyphosphate on the dry weight of the pulp. The resulting treated pulp crumbs were then passed into a hot air stream, entering the air stream at the inlet side of the drier where the air was at a temperature of 800 F. The pulp was carried in the hot air stream until it was dried to a solids concentration of This required a residence time of 4 minutes. The pulp was then separated from the gas stream in a cyclone separator, the air as it left the drier being at a temperature of 200 F. The dried pulp was collected. The pulp brightness following this treatment was 65.4%, whereas a sample which was treated in the same way as this except that our alkaline hydrogen peroxide bleaching agent was not used, following drying had a brightness of only 60.5%, or 2.5% lower than the brightness of the initial pulp.

Examples 2 to 15 The procedure of Exmaple 1 was followed, with the exception that the aqueous alkaline hydrogen peroxide solution introduced into the pulp in the disc mill was modified. The amounts of hydrogen pe oxide and alkali,

employed in these examples, and experimental results are shown in Table l which follows:

sample was treated in the same way except that an alkaline hydrogen peroxide bleacln'ng agent was not used.

TABLE 1.BRIGHTNESS OF PULP IMMEDIATELY AFTER BLEACHING- DRYING. WOOD SPECIES, SPRUCE, FIR GROUNDWOOD. CONTROL BRIGHTNESS, NOT HOT-GAS DRIED, 63.0

Percent alkaline additive Sodium Silicate Percent H202 1 NaOH 1 41 Be STPP 3 Other G.E. brightness 1 Hydrogen peroxide.

2 Sodium hydroxide.

5 Sodium tripolyphosphate.

4 Tetrasodlum pyrophosphate. B Trisodium phosphate.

Sodium carbonate.

Example 16 (comparative) (a) A stone groundwood pulp comprising 60% spruce and balsam fir, having an unbleached brightness of 58%, was obtained as a slurry. The slurry was dewatered and mechanically thickened to a solids concentration of 50%, and flufied in a disc refiner. During fluffing an aqueous solution containing 1.0% of hydrogen peroxide, 2.5% of sodium silicate (41 B.), 1.0% of sodium tripolyphosphate and 1.0% of sodium hydroxide was fed into the pulp to provide 0.3% of hydrogen peroxide, 0.75% of the sodium silicate, 0.3% of sodium tripoly phosphate and 0.3% of sodium hydroxide based on dry pulp weight.

In this case the treated, fluifed pulp was dried in a gas stream introduced at 650 F. and produced by burning Bunker C oil. This oil contained 3% of sulfur, which provided 16.4 lbs. of $0 in the gas stream. The gas left the drier at 200 F. and the pulp contacted it for 3 minutes.

The pulp was dried by this process to a solids content of 85%. Its brightness was lowered by the treatment to 57.5%.

(b) The process of this Example 16(a) was repeated with the exception that the oil burned to produce the hot gas was a Grade 1 fuel oil having a sulfur content of less than 0.5%. The pulp dried with this essentially sulfur-free hot gas had a brightness of 63 Example 17 A refiner groundwood pulp comprising cottonwood and pine and having a brightness of 63% was obtained as a slurry in water. It was dewatered and mechanically thickened to a solids concentration of and fiuffed in a disc refiner. A bleach liquor comprising an aqueous solution of 3.0% of hydrogen peroxide and 6.0% of sodium tripolyphosphate was introduced into the pulp at the entry point of the disc refiner in an amount to provide 0.85% of hydrogen peroxide and 1.75% of sodium tripolyphosphate in the pulp.

The pulp was then contacted with hot air, introduced at 850 F., for 4 minutes and thus dried to 85% solids. Results of this run, including the effect of storage on pulp brightness are given in Table 2 which follows. A second The brightness of this untreated pulp is also given in Table 2.

TABLE 2.PULP B RIGHTNESS Pursuant to the requirements of the patent statutes, the principle of this invention has been explained and exemplified in a manner so that it can be readily practiced by those skilled in the art, such exemplification including what is considered to represent the best embodiment of the invention.

What is claimed is:

1. A method of simultaneously rapidly drying and bleaching mechanical wood pulp having a high content of lignin and other non-cellulosic matter, comprising, dewatering said pulp to a solids concentration of 20 to 50% by weight and a water content of to 50% by weight, flufiing said pulpto provide essentially separate to loosely agglomerated pulp fibers, introducing an aqueous alkaline hydrogen peroxide solution containing 0.1 to 4% of hydrogen peroxide into said fluffed pulp, passing said treated fluffed pulp together with a heated gas stream which is at a temperature of 500 to 1000 F. and substantially free of reducing agents into a drying and bleaching zone, containing said treated fluited pulp with said heated gas stream in said drying and bleaching zone for 2 seconds to 10 minutes to dry and bleach said pulp, and thereafter removing the resulting bleached and dried pulp at a solids content of 65 to by weight.

2. The method of claim 1 in which the hydrogen peroxide is introduced in an amount of 0.5 to 2%.

3. The method of claim 1 in which there is added to said pulp an alkali in an amount of 0.5 to 5% by weight based on the dry weight of pulp.

4. The method of claim 3 in which the alkali is an alkali polyphosphate.

5. The method of claim 3 in which the alkali is an alkali orthophosphate.

6. The method of claim 3 in which the alkali is an alkali polyphosphate and an alkali orthophosphate.

7 8 7. The method of claim 1 in which the treated fiufied FOREIGN PATENTS pulp is in the drying and bleaching zone for 10 seconds 232 954 2/1961 Australia to 5 minutes. 2

References Cited 6 4, 44 5/1949 Gleat Butqm.

UNITED STATES PATENTS 5 S. LEON BASHORE, Prlmary Exammer 2,492,047 12/1949 KBurg et a1. 16271 X CL 3,055,795 9/1962 Eberhardt 162-100 8111; 3412, 19; 162-78, 100

3,316,141 4/1967 Bergholm et a1 162100 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,422, 99 Dated anuary 27, 1970 Inventor(s) Robert R. Kindron and Frederick J. Rosebush It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 3, "Tne" should read --'Ihe.

Column l, line 72, "Exmaple" should read -Example-.

Column 5, Table 1, Ex. 6 "l. 0 Na CO should read --l.ONa CO Column 6, line 60, "containing" should read contacting--.

Signed and sealed this 20th day of April 1971.

(SEAL) Attest:

EDWARD I'I.FLETCHER,J'R. WILLIAM E. SCHUYLER, JR. Attesting Officer Commissioner of Patents FORM ($59) USCOMM-DC 60376-F'69 Q I] S GOVIIIIIMEHT 'IIMTIIG OFHCE i... D1-3J 

